1. Field of the Invention
This invention relates to an image fiber applied to an endoscope or the like and, in particular, to an improvement of an integrated image fiber as it is usually called, in which a plurality of fiber elements, after being inserted in a jacket pipe used as a jacket layer, are drawn together to thereby have a common cladding in common.
2. Description of Related Art
Integrated image fibers of this type, when compared with flexible image guides known as fibers using acid-soluble glass, have the advantages that the ratio of the cross sectional area occupied by cores having a common cladding to that of the cladding (which is hereinafter referred to as a core area ratio) can be increased and when the image fiber is reduced to a very small diameter, fiber elements constituting the cores are hard to break. The integrated image fibers have thus found relatively many applications in non-flexible endoscopes and flexible endoscopes of very small diameters used for blood vessels, for instance.
The integrated image fibers are constructed so that an image transmitting portion composed of a plurality of cores and a cladding surrounding the cores transmits an image formed at the entrance end of the image fiber to the exit end thereof. In this construction, the core area ratio is usually set at approximately 20-50%.
In the integrated image fibers mentioned above, when advances are made in a reduction of the outer diameter and a high density of pixels of the image to be transmitted, spaces between the fiber elements constituting the cores are necessarily diminished. Hence, there is the need to prevent the generation of cross talk of light transmitted through the cores, and the thickness of the cladding with which the cores are covered must be ensured to some extent. This tends to increase the ratio of the cross sectional area occupied by the cladding to that of the image fiber (namely, a cladding area ratio). Since, on the other hand, light incident on the cladding becomes flare which deteriorates the quality of image at the exit end, it is necessary to eliminate the light propagating through the cladding as far as possible.
In the case of the prior art, to solve these problems, for example, Japanese Utility Model Preliminary Publication No. Sho 59-98402 discloses the method of removing flare attributable to cross talk by defining the refractive indices of the cores, cladding, and jacket layer of an image fiber. Further, the techniques of impregnating the outermost coating layer with a light absorption material are disclosed by Japanese Utility Model Preliminary Publication No. Hei 2-123904 and U.S. Pat. No. 4,615,583.
The prior art described above, however, cannot necessarily make an effective elimination of flare involved in the light propagating through the cladding in an image fiber which is relatively large in outer diameter or relatively small in length. Thus, endoscopes using such image fibers possess the drawback of remarkably degrading the quality of an available observation.